Cellular radio communications systems are widely used throughout the world to provide telecommunications to mobile users. A geographic area covered by a cellular radio system is divided into cells, each containing a cell site, through which subscriber units, such as mobile stations, communicate.
In general, an object of cellular radio communications system design is to reduce the number of cell sites required by increasing their range and/or capacity. This is because cell sites are expensive, both because of the equipment required and because of the need for a geographical site for each cell site. Geographical sites may be costly and may require extensive effort to obtain planning permission. In some areas, suitable geographical sites may even not be available.
The communications ranges in many systems are uplink (mobile to cell site) limited because of the limited power available at the subscriber unit, which may be a hand-portable subscriber unit. However, any increase in range would mean that fewer cells would be required to cover a given geographical area, thus advantageously reducing the number of cell sites and associated infrastructure costs.
When a cellular radio system is set up in an area of high demand, such as a city, then cell site communications capacity, rather than range, usually limits cell size. An increased cell site capacity would therefore reduce the required number of cell sites and so reduce costs, or for the same cell size, would deliver increased revenue from call charges.
After a cellular radio system has been set up, demand may increase to exceed the capacity of the existing cell sites. A method of upgrading existing cell sites to increase capacity where required might then reduce costs because the capacity of the system could be increased without acquiring any new geographical sites for cell sites or installing a greater number of cell sites.
One approach to increasing range and/or capacity, or to upgrade a cell, is to use directional antennas at a cell site physically to separate radiations at similar frequencies. This is known as sectorisation. It has been proposed to use three-sectored cells, having three antennas with nominally 120.degree. azimuthal beamwidth, or hex-sectored cells, having six antennas with nominally 60.degree. azimuthal beamwidth (as described for example in U.S. Pat. No. 5,576,717). In each case, one effect of the sectorisation is to reduce interference from mobiles and cell sites in adjacent and nearby cells, and thus to increase the total range and/or capacity of the cell site in a sectored cell relative to a cell using an omni-directional antenna.
However, there are problems which arise from the sectoring approach, particularly as the number of sectors increases. In any cellular system, a subscriber unit may move from one cell to another, necessitating transfer of the communication link from one cell site to another by a process known as handoff. In a sectored cell, a subscriber unit may also move from one sector to another, necessitating additional handoffs between the sectors of a cell site. Clearly, as the number of sectors increases, so does the number of handoffs, making increasing demands on the processing and communications capacity of the system.
A particular problem which is exacerbated as sectorisation increases is that a sectored cell site antenna is designed to produce a particular beam shape to cover its sector but may also produce sidelobes and backlobes, including elevation sidelobes and backlobes. These are likely to fall within sectors covered by the principal beams of other antennas at the same cell site, in which case they may be termed out-of-sector sidelobes and backlobes. (In this context, and throughout this document, the term principal beam is used to mean either a main beam or a diversity beam of a sector or a cell). As sectorisation increases, each antenna in a cell must be designed to form a beam having a decreased angular azimuthal width. This makes it more difficult for the designer to control the sidelobes and backlobes of the antenna. Also, as sectorisation increases, it becomes more likely that sidelobes and backlobes will fall within sectors covered by other antennas because there are more, narrower, sectors surrounding the cell site.
This aspect of sectorisation can cause a problem when a subscriber unit moves, within a first sector, from the principal beam of a first antenna covering the first sector into an out-of-sector sidelobe or backlobe of a second antenna, the principal beam of which covers a second sector. First, this may lead to an unexpected handoff between sectors (which may be non-adjacent), where in fact no handoff may have been necessary or desirable. Second, if the subscriber unit was communicating via the principal beam of the first antenna at a point where the principal beam gain is low, then it will have been transmitting at high power. When the subscriber unit then moves into the out-of-sector sidelobe or backlobe of the second antenna, the signal received by the second antenna may be very powerful and may interfere with or even swamp existing communications from other subscriber units to the second antenna. The subscriber unit may hand off to the second antenna, after which a power control signal can be transmitted from the cell site to reduce the subscriber unit transmission power, but until then, communications between the second antenna and other subscriber units may be adversely affected (this is known as the "near-far" effect).
One mode of communication used in cellular radio systems in which this problem may be particularly acute is spread spectrum communication, such as code division multiple access (CDMA). In such systems, all cell site transmissions, both in different sectors and in different cells, may be in the same frequency band.
This means that a subscriber unit moving from the principal beam of one antenna into an out-of-sector sidelobe or backlobe of a second antenna will always be transmitting on the same frequency as subscriber units already communicating via the second antenna, exacerbating the problem of interference (swamping) described above.
The description above assumes for simplicity that each principal beam covering a sector is generated by a separate antenna. However, in some sectored cells, a single antenna may generate the principal beams covering more than one sector. In that case, depending on the handoff mechanism between sectors, a similar problem may arise if a sidelobe or backlobe of one sector overlaps a second sector generated by the same antenna.